In current optical communication systems, signals are transmitted long distance using multiple wavelengths of light passing through optical fibers. Each optical carrier wavelength can be encoded with a unique set of information. The broader the optical bandwidth of the transmission system, the more information can be transmitted using more wavelength-division multiplexed (WDM) signals. Such WDM optical systems use optical fibers, which produce some level of optical loss, typically 0.15-0.3 dB/km. Additionally, components used in these systems to perform functions such as dispersion compensation or dynamic equalization add optical loss. In order to overcome these losses and maintain the optical signal to noise ratio (OSNR) of each channel, optical amplification is required periodically. Such optical amplification must be broadband, at least as broadband as the wavelength range of signals to be transmitted and its gain must be close to constant for all signal wavelengths (gain flat) so that all signals experience nearly the same gain. Additionally, the amplification must not add much noise to the amplified signal, as represented by a low amplifier noise figure (NF).
Unfortunately, the gain of most optical gain media is not flat across a wide range of optical wavelengths. However, flatness can be achieved using an optical filter, which is a device that creates a predetermined wavelength-dependent optical loss to perfectly compensate for any gain flatness error. Such a filter is typically placed within each amplifier to achieve gain flatness to some tolerance level. For most optical gain media, such a filter makes the gain flat at only one particular gain magnitude. So, a different filter is needed if the optical gain or output power level of the amplifier changes.
While optical gain is possible in many different gain media, in most current deployed optically amplified communication systems, the gain medium consists of erbium ions doped into a silica-based fiber. Such erbium-doped fiber amplifiers (EDFAs), when provided with sufficient optical pump radiation from available pump diodes, can provide efficient low noise amplification at the low loss window of optical transmission fibers, namely near 1550 nm. EDFAs can produce gain across a 40 nm window from 1525-1565 nm (called the C-band) or can be designed differently to produce gain from 1565-1605 nm (called the L-band). In both bands, the gain is not adequately flat for most WDM optical communications systems without the inclusion of some filtering, and the shape of the gain varies with operating condition.
In most cases, optical systems contain a wide range of optical span lengths with a range of component losses, leading to an even wider range of optical losses. These must be compensated by EDFAs that achieve gain flatness for a wide dynamic range of optical gain levels. Such variation can be accommodated in several ways. The most direct way is to design a different custom amplifier, typically an EDFA, that is gain flat, produces a low NF and adequate output power for each prescribed operating gain point. Such an approach meets performance needs, but is expensive and requires a large inventory of EDFAs designed to different specification (often called design codes). A second approach is to add loss to every span to make all span losses equal and to emulate the maximum component loss ever present in a worst case span, hence requiring all amplifiers to be the same. Such an approach unnecessarily and often severely degrades the NF and/or power output of the EDFAs and the OSNR at the end of the system.
The third and prevailing approach to accommodate gain variation in optical amplifiers is to add a variable loss element, typically called a variable optical attenuator (VOA) within each amplifier at a location where it does not unnecessarily penalize the NF or power output. Such VOAs are commercially available and have been made using a variety of optical technology platforms. Using a VOA within an EDFA, the operating gain can be adjusted by changing both the pump power used and the loss setting of the VOA so that a low NF and gain flatness can be maintained for a range of gain levels and output powers. The range of gain levels (the dynamic range) that can be accommodated while still maintaining adequate performance (including a low NF, gain flatness, and required output power) by using such a VOA approach is typically less than 15 dB. Additionally, some of this dynamic range is often used to adjust for changes as the system ages, so that the useful dynamic range to adjust for link variations is typically less than 10 dB.
The usable dynamic range of an EDFA is often further reduced in order to accommodate a range of lossy component modules, known as dispersion compensation modules (DCMs). The loss of such modules, and the need for their use, depends on the bit rate of the system, the length of the span fibers and the type of transmission fiber used. Depending on the system design, as little as 3 dB of amplifier dynamic range might be available to accommodate span length variation, even when a VOA is included in each amplifier. This usually means that, even with a VOA in each EDFA, several EDFA codes are required to adjust for all possible system link scenarios.
Even further, systems designed to make different length links and to accommodate different types of traffic require even more EDFA codes. For example, most optical system vendors support different designs for long-haul (LH) systems that convey information up to 1000 km in point-to-point links, metro systems that send information around a ring like architecture of a few hundred km lengths and ultra-long-haul (ULH) systems that send information across transcontinental distances.
The proliferation of EDFA designs and unique incompatible optical modules is a great expense and reduces flexibility to accommodate future needs without large-scale redesigns. Accordingly, it would be desirable to design an optical system that does not require a large number of EDFA codes or unique optical modules.